Many different varieties of retroreflective sheets (sometimes referred to as being "light-retroreflective" or as "sheetings") have been proposed and marketed in the past for a variety of purposes.
Retroreflective sheets are perhaps most commonly used for safety purposes, specifically for road signs and as stickers, emblems, and medallions for vehicles, e.g., bicycles and automobiles, and personal articles, e.g., clothing, bags, etc. In such applications, it is typically desired that the sheet provide very strong retroreflective brightness for particular observers, e.g., drivers of approaching vehicles. As a result, such sheets are typically constructed to minimize light dispersion such that incident light is reflected only in the narrow range in the direction of the light source, i.e., retroreflection. Because of this, such sheets are readily illuminated by approaching vehicles with headlights in operation, making them most readily visible to occupants of those vehicles, but are not as effectively illuminated by other light sources. For example, when the light source is the sun, a street lamp, or indoor lighting, light retroreflected light by the sheet is not readily recognized or detected by most observers except for those who happen to positioned substantially adjacent the path of the light from the illumination source to the sheet. This problem is particularly relevant in the case of street advertisements, storefronts and window displays, station signs and other various decorative and related purposes other than for the safety purposes.
A decorative film suited for such uses as signs and traffic safety equipment is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-45507. As shown in FIG. 1, this decorative film comprises protective layer 36, glass beads or microspheres 32 partially embedded in protective layer 36 and binder 31, reflective thin film 33, support layer 35, adhesive layer 37, and release liner 38. The thickness of binder 31 varies. Glass beads 32 have a diameter of about 500 microns (".mu.m") or lower, a refractive index of 2.0 or higher, and an embedding ratio in anchoring binder 31 to about 10 to about 80 percent of their diameter. Reflective thin film 33 consists of a metal vapor deposited thin film which contacts glass beads 32 in depressions 34. These decorative sheets have the effect of changing color tone depending on the angle viewed under constant light irradiation, and shining beautifully in a rainbow dispersion spectrum. However, the sheets are limited to retroreflection of light which is incident at angles within a relatively narrow incidence angle range, and thus it has been difficult to obtain its excellent decorative effect at other incidence angles.
Examples of retroreflective sheets with similar decorative effects are the retroreflector described in Japanese Unexamined Patent Publication (Kokai) No. 2-54922 and the retroreflective sheet material described in U.S. Pat. No. 5,503,906. Both of these references disclose a patterned decoration to be formed by providing a transparent or colored resin layer over parts of the front or back side of an exposed-bead reflective sheet. Although such sheets can provide colored retroreflection, they do not provide rainbow decoration.
Patterned or selective reflected light may be obtained by the method for producing a retroreflective pattern described in Japanese Unexamined Patent Publication (Kokai) No. 60-128401. According to this approach, the reflective coating side of the reflective sheet is contact bonded to an adherend partially coated with an adhesive in a pattern, and only those sections serve as the retroreflective sections. However, as with the technique described above, rainbow decoration cannot be achieved.
In addition, the vehicle reflector described in Unexamined Utility Model Publication (Kokai) No. 6-78076, the sheet with a retroreflective side described in Japanese Unexamined Patent Publication (Kokai) No. 2-140703, and the method for producing a retroreflective sheet described in Japanese Unexamined Patent Publication (Kokai) No. 48-72290 represent techniques for achieving wide angle retroreflection, i.e., the capability of retroreflection even when the incident angle relative to the sheet is relatively acute. That is, these techniques allow retroreflection across a wide range of angles by causing light to be retroreflected by sheet 10 when incident at an acute angle .theta..sub.1, .theta..sub.2 with respect to sheet 10 as shown in the attached FIG. 2.
U.S. Pat. No. 3,801,183 discloses retroreflective films bearing retroreflective legends and backgrounds that comprise, in part, a discontinuous varnish layer.
The need exists for retroreflective sheets that provide effective retroreflective effect over a wide variety of incidence angles and that also provide decorative effects that are visible under a variety of viewing conditions.